1. Field of the Invention
This invention relates to water resistant ink compositions which are highly suitable for use in recording methods that employ aqueous inks, and especially ink-jet printing. These compositions, when printed onto a recording medium and dried, form printed images which experience no loss whatsoever in quality even upon accidental exposure to water or beverages. Moreover, they have excellent color stability.
2. Prior Art
Ink-jet printing makes use of a number of different ink discharge techniques to form droplets of ink and deposit some or all of the droplets on a recording medium such as paper (the term "ink" being used here to refer broadly to recording liquids). Examples of such techniques include electrostatic absorption, the application of mechanical vibrations or changes to the ink using a piezoelectric element, and thermal bubble formation from the ink combined with use of the resulting pressure. These ink-jet printing methods are notable because they generate little noise and enable high-speed, multicolor printing.
The inks commonly used in ink-jet printing contain water as the main component to provide safety and good recording characteristics. Polyhydric alcohols are also included to prevent clogging of the nozzles in the ink-jet printing system and to enhance discharge stability.
In order to keep pace with the better performance and more widespread use recently of ink-based recording systems and related technology, further improvement needs to be made in the properties of ink compositions. This need for improvement is particularly acute with regard to the water-resistance of inks. The past few years have seen a very rapid rise in the use of ink-jet printers, both in the office and at home. However, in the home and office environment, there is a greater chance of printed matter coming into contact with water or beverages such as coffee or soda. Under such conditions, it is essential that the ink be water resistant.
Efforts have been made to use pigment-containing inks because of their excellent water-resistance. A major drawback has been the inferior color development of printed images obtained with pigment-containing inks compared with printed images obtained with dye-containing inks.
Very few if any of the water-soluble dye-containing inks with good color development currently on the market are water resistant. So when a printed image comes into contact with water, for instance, the image is almost always ruined by color bleeding or the like. Because both the dye itself and the polyhydric alcohol added to improve the discharge stability of the ink are water-soluble, direct contact of the printed image with water allows these components to dissolve out into the water, destroying the quality of the image.
Of course, the polyhydric alcohol component of a dye-containing ink may be excluded to provide better water-resistance, but this results in a poor discharge stability. An attempt to confer reactivity to the dye itself so that it bonds with the substrate is reported in Shikizai (Coloring Matter), 67, 6, 356-361 (1994). This attempt consists of introducing alkoxysilyl groups onto the dye molecule to make it reactive. Unfortunately, it is difficult to successfully incorporate alkoxysilyl groups since polar groups such as --SO.sub.3 Na, --NH.sub.2 and --CN are normally included in water-soluble dyes to confer water solubility, and these polar groups tend to react with the alkoxysilyl groups. Moreover, the resulting dyes have a poor stability in aqueous solutions, and are also unduly expensive.
Another method is described in Shikizai, 66, 9, 517-522 (1993), wherein a dye is added to tetraethoxysilane or methyltriethoxysilane to form a sol. A glass substrate is then treated with the sol to immobilize the dye. However, a major disadvantage of this approach is that such a sol gels immediately upon addition to a basic aqueous ink, and thus cannot be used in aqueous inks.
Another approach commonly taken is to use silicone compounds as additives for imparting water-resistance. Such compounds do indeed confer excellent water-resistance in solvent systems, but their stability in water is rather poor. At best, they are somewhat stable under weakly acidic conditions, but generally lose all stability when the system is even mildly alkaline. Given that water-soluble dyes are most often alkaline, silicone compounds are of no use in such a system.
Water-soluble polyamines have been used in quite a few cases recently to provide water-soluble dyes with water-resistance. However, one drawback has been intense color changes which alter the color tone of the dye, probably due to the decomposition of azo groups in the dye on account of the reactivity and basicity of the amino groups, and especially primary amino groups, on the polyamine.
JP-A 6-279678 discloses an aqueous surface treatment composition in the form of an aqueous solution which contains as a primary component either the hydrolyzate of the product obtained by reacting an aminotrialkoxysilane with an organic compound that reacts with the amino group on the silane, or the product obtained by then reacting this hydrolyzate with an organometallic compound such as a tetraalkoxysilane. Although blocking of the amino group limits changes in the color tone to some degree, the use of polydiglycidyl compounds as the blocking agent destabilizes the solution itself, such as by making it subject to gelation. This method also calls for the addition of an anionic surfactant as the stabilizer, which can be detrimental to the water-resistance. Another problem has been the instability of the solution under alkaline conditions, making it of little use as an additive in water-soluble ink compositions.
The applicant proposed, in JP-A 10-212439, a water resistant ink composition comprising an organosilicon compound prepared by hydrolyzing components (A) and (B) used in the present invention. However, when used as magenta or black inks, this composition does not have sufficient stability at relatively high temperatures.